2,3-Pyridinedicarboxylic acid, which is also called quinolinic acid, has a wide variety of uses as intermediates in the production of medicinal or agricultural chemicals, dyes, pigments, and the like.
A number of methods with respect to the preparation of 2,3-pyridine-dicarboxylic acid have been investigated. There have been proposed, for example, a method in which quinoline is oxidized in sulfuric acid with hydrogen peroxide or sodium chlorate in the presence of a ruthenium compound as a catalyst (JP-A 03-271275), a method in which quinoline or 8-hydroxyquinoline is oxidized with hypochlorite ions in the presence of a ruthenium compound as a catalyst (JP-A 61-212563, JP-A 02-83370, and JP-A 03-101661), a method in which quinoline is oxidized with chlorite ions in the presence of a ruthenium compound as a catalyst (JP-A 03-157371), a method in which quinoline is oxidized with oxygen in the presence of a large amount of cobalt ions (JP-A 49-55673), and a method in which 8-hydroxyquinoline is oxidized with chlorate ions in the presence of vanadium ions (JP-A 58-105964).
A method in which quinoline is oxidized with sodium chlorate under acidic conditions in the presence of an equimolar amount of a copper (II) salt is described in JP-A 62-209063. Another method in which quinoline is oxidized in two steps using a chlorate salt in the final oxidation step is described in JP-B 62-18551. Other known methods include a method in which quinoline is oxidized with hydrogen peroxide in the presence of copper (II) ions [Chem. Ber. 65, 11 (1932)], a method in which 8-hydroxyquinoline is oxidized with nitric acid [Chem. Ber. 12, 983 (1879)], and a method in which quinoline is oxidized with ozone [Synthesis, 11, 880 (1989)].
As an improvement on the method described in Chem. Ber. 65, 11 (1932) in which quinoline is oxidized with hydrogen peroxide in the presence of copper (II) ions, it is described in JP-B 60-54305 that an increased yield is attained by performing the reaction under acidic conditions with sulfuric acid, collecting the resulting quinolinic acid copper (II) salt by filtration, and recirculating the filtrate for repeated use as a reaction medium for the oxidation reaction.
As described above, a great number of methods have been proposed with respect to the production of 2,3-pyridinedicarboxylic acid by oxidation of quinoline or its derivative. Seen the other way around, this indicates that there is no method which has been established for commercial production of 2,3-pyridinedicarboxylic acid.
At present, 2,3-pyridinedicarboxylic acid is commercially produced by a method in which quinoline or 2,3-dimethylpyridine is oxidized with ozone or by a method in which 8-hydroxyquinoline is oxidized with a hypochlorite or nitric acid. However, these methods have problems, such as that the costs of the raw material or equipment are high, the yield is low, and/or waste liquor, which requires cumbersome treatment, is discharged in a large amount.
The present invention provides a process for producing 2,3-pyridine-dicarboxylic acid in which the above-described problems are alleviated so that the process is suitable for application to commercial production.
As is known to those skilled in the art, mother liquor remaining after precipitated 2,3-pyridinedicarboxylic acid has been separated out still contains a considerable amount of 2,3-pyridinedicarboxylic acid, but this mother liquor has not been utilized in an effective manner. This is one of the causes for the low yields in the prior art methods. In addition, the entire mother liquor is treated as waste liquor, thus requiring increased costs for treatment of waste liquor.
If the mother liquor is recirculated to the step of preparing 2,3-pyridine-dicarboxylic acid, the 2,3-pyridinedicarboxylic acid contained in the mother liquor can be effectively utilized while the costs of waste liquor treatment can be restrained. However, the mother liquor also contains, in addition to 2,3-pyridinedicarboxylic acid, significant amounts of organic by-products formed during the synthesis of 2,3-pyridinedicarboxylic acid. Therefore, use of the recirculated mother liquor in the preparation step leads to a significant decrease in the purity of the desired product, 2,3-pyridinedicarboxylic acid. This is thought to be the main reason why the mother liquor has not been positively utilized.
For example, in a typical process for producing 2,3-pyridinedicarboxylic acid via copper (II) 2,3-pyridinedicarboxylate, the copper (II) salt is decomposed by reacting with an alkali in solution to give a solution of an alkali metal salt of 2,3-pyridinedicarboxylic acid, which is then reacted with a mineral acid to precipitate 2,3-pyridinedicarboxylic acid. The precipitates are recovered as the 2,3-pyridine-dicarboxylic acid product.
With respect to the production of 2,3-pyridinedicarboxylic acid copper (II) salt by oxidizing quinoline with hydrogen peroxide in the presence of copper (II) sulfate, it is described in JP-B 60-54305 that the filtrate remaining after the copper (II) salt is recovered is recirculated to the step of preparing the copper (II) salt by the above-described oxidation of quinoline. However, the filtrate contains significant amounts of by-products, and use of the recirculated filtrate significantly deteriorates the purity of the product.
In JP-A 61-212563 which discloses a method for preparing 2,3-pyridinedicarboxylic acid by oxidizing quinoline with a hypochlorite and a ruthenium oxide catalyst in the presence of a large amount of sodium hydroxide, it is described that after sodium chloride is filtered off from the reaction mixture, an acid is added to the filtrate to precipitate 2,3-pyridinedicarboxylic acid and recover it, and 2,3-pyridinedicarboxylic acid remaining in the mother liquor, from which the precipitated product has been separated, can be recovered as its copper (II) salt by adding copper (II) oxide to the mother liquor. However, in this process, the 2,3-pyridinedicarboxylic acid copper (II) salt recovered from the mother liquor is contaminated with a large proportion of impurities since the mother liquor contains organic impurities in an overwhelmingly larger amount than 2,3-pyridinedicarboxylic acid. Therefore, the recovered copper (II) salt produces 2,3-pyridinedicarboxylic acid with low purity.